The Film

The silver salts, silver bromide, silver chloride and silver iodide are
light sensitive and discolour when they are exposed to light. In a photographic
film these salts are suspended in gelatine to form the photographic emulsion.
In earlier days the emulsion was coated onto glass plates and later onto
celluloid. Unfortunately celluloid burns easily and today a nonflammable
base of acetate or polyester is used to give us the "safety film".

When
a photographic emulsion is exposed to light, the silver halides change
to photo-activated silver and bromide atoms. The silver is said to be
triggered. The information contained in the emulsion is very small, not
visible and referred to as a 'latent image'. During development more silver
halides are reduced chemically to silver and the 'latent image' is intensified
and becomes visible. The remaining silver halides are removed during the
fixing process.
The parts of the film that receive a lot of light will contain much silver
after development and appear dark. The parts that did not receive any
light will remain clear. The film will be dark were the subject was light
and light were the subject was dark - in other words: we have a negative
image on film. By exposing a photographic paper or film through this negative
we get a paper or film positive.

1.

Film
Characteristics
Some film characteristics are described below: a. Cross-section of Film
A cross-section of film, examined under the microscope, would look
like this:

The thickness of the emulsion varies from 5µm to 20µm,
depending on the type of film and its sensitivity. Films with a
low ISO rating have a thinner emulsion than films with a high ISO
rating.

b. Grain
If we look at an exposed and developed film under the microscope
we see the dark specks of silver. These specks are called the 'grain'
of the film and the size of the film grain depends on:

The film speed: slow films have smaller, same sized grains and
fast films bigger grains of varying sizes.

The developer used to develop the film.

The method of agitation and total 'wet time' of the film.

c.
Colour sensitivity
A narrow band of electromagnetic waves with wave lengths from 400-700
nm is seen by the human eye as light. The eye is the most sensitive
to wavelengths of 570 nm, that of yellow-green light. Reds and blues
are perceived as dark colours.
This is shown in the figure below.

The
silver halides (silver salts) used in photographic emulsions are
only sensitive to ultra-violet and blue light. To make film 'see'
colour like the human eye it has to be sensitized for other colours
as well. By using colour couplers, film is sensitized for green,
red and even infrared. Emulsions without any colour couplers are
called as 'ordinary' or 'non-colour sensitive'. Films that are sensitized
for green are called 'orthochromatic' and those sensitized for green
and red are called 'panchromatic'.

d.
Density
When light falls on a developed negative, part of the light will
be transmitted through the negative and part of the light will be
reflected or absorbed by the negative. The relation between the
light incident on a piece of exposed film and the light transmitted
through that piece is called the opacity of the film. The log10
of the opacity is called the density of the film. Density is used
as a measure of the 'blackness' of a tone on a processed negative.
Black parts of a film have a high density and clear parts a low
density.

e.
Contrast
The range of grey tones an emulsion is can produce, from complete
transparency to darkest black, is called the contrast of the material.
A small range of grey tones is called 'high contrast' and a large
range of grey tones is called 'low contrast'. Plotting the density
of different grey tones against the necessary exposure, results
in the following graph:
In the 'high contrast' material a few big 'steps' are taken to go
from A to B. But in the 'low contrast' material many small steps
are taken from A to B. The gradient of the straight part of the
curve is called the 'gamma' or contrast index.
Negative contrast is inherent in the film material, but can be considerably
influenced through exposure and development. Films used for pictorial
photography are normally developed to a contrast index of 0.6 to
0.8.

f.
The exposure index
The sensitivity of an emulsion to light is called the 'speed' of
the film or paper. Films with a high sensitivity are called 'fast',
films that require more exposure are called 'slow'. To make comparisons
between films and papers possible, they are given a 'speed rating'.
There are two standards:

Arithmetic: The number indicating the speed is doubled when the
sensitivity doubles.

Logarithmic: For each doubling of speed, the rating is numerically
increased by three.

Both ratings are given on any film cartridge under the new ISO
(International Standard Organization) rating and are written as
follows: ISO 100/21 or ISO 400/27. In the table below some of
the old ASA (American Standard Association) and DIN (Deutsche
Industrie Norm) values are given as well as the new ISO.

ASA

25

50

100

125

200

400

800

DIN

15

18

21

22

24

27

30

ISO

25/15

50/18

100/21

125/22

200/24

400/27

800/30

g.
Reciprocity Failure
The effective film speed of an emulsion is reduced for exposures
longer than 1 second or shorter than 1/1000 s. This results in underexposure
even if the exposure meter reading was adhered to. Possible corrections
can be looked up in the technical information on the film available
from the manufacturer. The adjoining graph can be used as a starting
point to calculate the corrected exposure time for exposures longer
than 1 second.

2.

Film Processinga.
Chemicals
All chemicals used in processing photographic films and papers should
be handled with care! Always read instructions on the labels and take
the precautions advised. Mix chemicals in well ventilated areas, keep
your hands out of the solutions and wear protective gloves if necessary.
Do not eat, drink or smoke in the darkroom.

Developers

Developers are available in powder form or as a liquid concentrate.
Make up powdered developers exactly as instructed and let stand
for 24 hours before using them. Never mix part of a batch and
always add chemicals in the sequence the appear in the recipe.
Used developer may be poured down the drain.

Stop-baths

Commercial acid indicator stop-baths should be mixed according
to the instructions on the label. They normally have a light yellow
colour that changes to purple when the solution is exhausted.
You also can mix your own stop-bath of 2% glacial acetic acid.

Fixers

Most fixers are liquid concentrates. Mix according to instructions
and note what fixing times are given for the dilution used. To
test a fixer, place a piece of undeveloped film into the fixer
and note the time it takes to clear. Double that time will be
the correct fixing time (tf) for the film.

Note:Never pour exhausted fixer down the drain. Store it in
a bottle and arrange for silver recovery firm to collect your
exhausted fixer. They will even pay you for it.

b.
Development Procedure

Prepare the developer, stop bath (2% - 3% acetic acid) and fixer
according to the manufacturers instructions and check their temperatures.
Use a water bath to bring the temperature within the 20°C -
25°C range. Note developing and fixing times. If the temperature
is higher or lower than the recommended temperature, find the correct
time from the chart on page 10 and set the timer (td).

In complete darkness insert the exposed film
into the dry spiral. Place the spiral into the tank and close
it properly.
You may then continue in the light.

Pour the developer quickly into the tank and
start the timer. Close the lid of the tank and knock the bottom
twice to get rid of any air bubbles sticking to the film.

Invert the tank, i.e., turn it upside down and
then right side up, 4-6 times in the first ten seconds of every
minute for the rest of the developing time.

Pour out the developer ten seconds before the
end of the developing time.

Pour stop bath into the tank and close the lid.
Invert tank 6 times, let stand for 15 seconds, invert 6 times
and pour out the stop bath. orRinse film in running water for 1 minute.

Pour fixer into tank, close lid and for 30 seconds
keep inverting the tank continuously, let it stand for 30 seconds.
Then turn it upside down 6 times every minute for the rest of
the fixing time.

When the fixing time is over, pour the fixer
into its storage bottle and wash the film using one of the following
three methods.

1. Place the tank under a water tap and wash
the film in running water for 20-30 minutes. The water temperature
should not be below 18C.

2. Wash the film in running water
for 1-2 minutes as in 1. Pour water out and pour a hypo clearing
agent or wash aid into tank. Agitate for recommended time. Pour
wash aid into its storage bottle and wash film in running water
for 5 minutes.
3. a. Fill tank with water and invert it five times. Pour water
out.
b. Refill tank and invert it ten times. Pour water out.
c. Refill tank for the third time and invert twenty times. Note: Use 500 ml per 36 exp. film. This method is recommended
by Ilford and does not necessarily apply to other makes of film.
Kodak's T-max films definitely have to be washed longer.

When you have finished washing the film, rinse the film in a wetting
agent (diluted according to instructions) for 1 minute and hang
it up to dry in a dust free place.
Thoroughly wash the tank, spiral and all other containers used and
allow to dry before storing.
Film development is a chemical process and therefore temperature
dependent. The higher the temperature, the shorter the processing
time. If your temperature differs from the temperature given in
the table, calculate the new development time from chart below.

Time-temprature development chart

1. Find the point representing the time at the temperature given
in the table.
2. Follow the diagonal line corresponding to this time to the point
where it cuts the horizontal line representing the temperature of
your developer.
3. Draw a line straight down from this point and read off the new
development time on the horizontal axis of the chart.

The
following table gives developing times for some films and developers
at 20C. These times are for normal contrast. If you use a diffusion
enlarger, increase the time. Agitation should be 4-6 inversions
in the first ten seconds of each minute, except for Rodinal -continuous
agitation for the first minute, then two inversions every thirty
seconds for the rest of the development time.
These times are only a starting point and each photographer should
do his own tests to decide on the best results. For consistent results
it is advisable to use processing times that are longer than 5 minutes.